The present invention relates to a torque control technique of an electric motor for driving an electric vehicle.
An electric vehicle is provided with an electric motor and a battery. The motor is driven by electric power supplied from the battery, so that the vehicle can run. To reduce power consumption in such an electric motor vehicle, it has been developed a technique by which a driving energy is converted to an electric energy by an electric motor (motor generator) during a deceleration thereby enabling regeneration-charging of the battery.
The electric vehicle is configured to calculate a motor torque requested by a driver based on information on, for example, an accelerator pedal opening, a vehicle speed and the like, and control a torque of the electric motor by controlling a motor controller (inverter) to generate a power-running (driving) torque during acceleration and a regeneration torque during deceleration.
Generally, a battery is limited in a usable range of a changing rate of the battery, which is related to an open end voltage of the battery. In a case that the battery is charged or discharged over the usable range, the battery may be deteriorated. Also, in a case that a battery current, which is a current flowing in the battery when the battery is charged or discharged, exceeds acceptable amount, the battery may be deteriorated.
A technique has been developed, which is capable of controlling a battery to prevent a charging rate of the battery or a battery current from exceeding the usable range (see Japanese Patent No. 4052080 and Japanese Patent No. 4200956).
In a control apparatus disclosed in Japanese Patent No. 4052080, a maximum level and a minimum level are set for a battery voltage or a battery current in accordance with a battery state such as a temperature of the battery when an electric motor drive is controlled.
In a control apparatus disclosed in Japanese Patent No. 4200956, charging and discharging time of the battery is limited in accordance with a temperature of the battery thereby preventing a charging rate of the battery from exceeding the usable range largely.
However, since a battery has an internal resistance, when the battery is charged and discharged, variation of voltage corresponding to an integrated value of the battery current and internal resistance is occurred. Accordingly, even though it is controlled in the same manner as disclosed, for example, in Japanese Patent No. 4052080 and Japanese Patent No. 4200956, if the operation of the electric motor is changed when the charging rate of the battery or the battery current is near the limit value, the charging rate of the battery or the battery current may exceed the limit level. For example, if the driving current of the electric motor is increased and the discharging current of the battery is increased to accelerate the vehicle when the charging rate of the battery is near the lower limit level, the battery voltage falls below the lower limit level and an over discharge may be occurred. Alternatively, if a vehicle runs along a downhill when the charging rate of the battery is near the higher limit level, at the time of regeneration charge, a rotational speed of the electric motor is increased and the charging current of the battery is increased, thereby the battery voltage may exceed the higher limit level and an over charge may be occurred.